PROCESS FOR THE PREPARATION OF METAL SULPHIDES

Abstract: The invention relates to a process for the preparation of metal sulphides by reacting corresponding metal oxides at temperatures between 500 and 1500 DEG C in a stream of gaseous CS2, characterised in that the CS2 is generated from elementary carbon and gaseous H2S by reaction at temperatures between 900 and 1500 DEG C upstream of the metal oxide. The process furnishes metal sulphides of high purity, thus being of specific usefulness in optical glass materials for, e.g., fibre optics.

Description: Process for the Preparation of Metal Sulphides The present invention relates to a process for the preparation of metal sulphides. Metal sulphides find various uses in technical and scientific applications. For in¬ stance, several rare earth and other metal sulphides are of specific interest as materials or components in optical glasses for the manufacture of, e.g., optical fibres and active fibre amplifiers. In this field of application there is an increased demand of new materials useful for fibre optics with improved optical properties. Typical of such materials are, for example, the sulphides of Lanthanum (La 2 S 3 ), Praseodymium (Pr 2 S 3 ), Holmium (Ho S 3 ), Gallium (Ga S 3 ) and Germanium (GeS 2 ). One of the main requirements to be met for optical fibre applications, specifically for 1.3 μm fibre optic systems, is ultra high purity of the sulphides, as it is expected that an increase in purity of the sulphides could lead to a substantial increase in efficiency of the fibre optics. Several methods for the preparation of metal sulphides are known which are based on the reaction of the metal or the metal oxide with sulphur, hydrogen sulphide or other sulphur containing reagents. For a review of methods for the preparation of rare earth metal sulphides, reference is made to Gmelin Handbook of Inorganic Chemistry, 8th Edition, Rare Earth Elements - C7, page 69-74, Springer Verlag, 1983. It is apparent that the methods reported there suffer from severe disadvantages, most of all limited applicability and ineffectiveness, and do not furnish products of high purity. For instance, high temperature reaction of the metal with sulphur in a sealed silica tube works for a limited range of metals only, where the product sulphide is miscible with excess metal, permitting a continuous reaction of the elemental charge. The partial formation of an insoluble product sulphide, such as La S 3 may cause the reaction to cease and leads to rupture of the ampule and explosion. High temperature reaction of the metal with hydrogen sulphide is similarly limited by the formation of insoluble surface sulphide which inhibits full progress of the reac¬ tion. High temperature reaction of the oxide with hydrogen sulphide and/or sulphur vapour is effective with elements such as Group Ila metals only, but may leave a high level of several percent of unreacted oxide impurity. It is ineffective in the case of many rare earth oxides. Moreover, many of the rare earth oxides require temperatures above 1300 °C in this type of reaction which is inconvenient for normal furnace systems. Carbon disulphide vapour was reported to be a more effective sulphiding agent for several rare earth metal oxides. CS 2 may be supplied into the reaction system by bubbling an inert carrier gas through a bottle of CS 2 liquid and passing the gaseous mixture into the reactor tube containing the oxide and which is heated to temperatures at about 1000 °C. This is an unpleasant and dangerous operation, since CS 2 is toxic and highly flammable. In situ generation of CS from sulphur and carbon in the reaction zone was also re¬ ported. Eastman et al., J.Amer. Chem. Soc, 72, 2248 (1950), reported the preparation of cerium sulphide from the dioxide by passing a stream of hydrogen sulphide over it in a carbon furnace at elevated temperatures. An intermediate formation of CS (CS ?) is speculated. In-house investigations on these methods now confirmed that CS is a more effective sulphiding agent. To avoid the dangerous handling of CS , in situ generation from carbon and hydrogen sulfide has been studied. In the experiments the metal oxide was contained in a graphite boat which was eroded at elevated temperatures by the hydrogen sulphide in the immediate vicinity of the oxide. Analytical investigation of the products so formed showed, that the metal sulphides contain carbonaceous impurities and/or contamination with binder materials from the graphite. It was, therefore, the task of the present invention to provide a process by which in a safe, simple and effective manner metal sulphides can be produced which are pure enough for the use in optical glass materials, specifically designated for fibre optics. It has been found out that these objects can be met by a process in which metal oxides are reacted at temperatures between 500 and 1500 °C in a stream of gaseous CS 2 , characterised in that the CS 2 is generated from elementary carbon and gaseous H 2 S by reaction at temperatures between 900 and 1500 °C upstream of the metal oxide. The object of the invention is therefore a process for the preparation of metal sul¬ phides by reaction of the corresponding metal oxides at temperatures between 500 and 1500 °C in a stream of gaseous CS , characterised in that the CS is generated from elementary carbon and gaseous H S by reaction at temperatures between 900 and 1500 °C upstream of the metal oxide. The process of the invention is based on the principle of in situ generation of CS from elementary carbon in a stream of gaseous H S at appropriate temperature, followed by the reaction of the so formed gaseous CS with the metal oxide yielding the corresponding metal sulphide. Here, in contrast to known methods utilising the same principle, the CS 2 is not generated in the intimate vicinity of the oxide but is formed at a separated location upstream of the oxide in a suitably designed reaction apparatus. The main advantage of this method is that metal sulphides are obtained which are free of impurities and contaminants stemming from the carbon and which show a re¬ sidual oxide content not exceeding 0.5 % by weight. As a rule, the residual oxide content of sulphides obtained with the process of the invention lies within the region of 0.01-0.1 % by weight. Thus, this process results in metal sulphides of extreme purity making them highly suitable as optical glass materials, specifically for optical fibre applications. Further advantages are the easy and safe practicability and the broad applicability of the process, as practically any metal sulphide is accessible from the corresponding oxide. The process is especially useful in cases where known processes are ineffective or even fail. The process of the invention is preferably applicable to the preparation of the sul¬ phides of Group Ha metals of the periodic system of the elements, especially the sulphides of Mg, Ca, Sr and Ba, of Group Ilia metals, especially of Al and Ga, of Group IVa metals, specifically of Si, Ge and Sn, of transition metals and rare earth metals. Most preferred is the preparation of the sulphides of La, Pr and Ho, namely La 2 S 3 , Pr 2 S 3 and H0 2 S 3 . The process may be carried out in any suitably designed reaction apparatus which al¬ lows a separate placement of elementary carbon and metal oxide, an appropriate heating of both locations to reaction temperature, and a continuous feeding of gaseous H 2 S coming into contact with the carbon first. In a preferred embodiment the reaction is carried out in a horizontally placed tube¬ like reactor in which in a first reaction zone a container is placed which is filled with pulverised elementary carbon and in a second reaction zone a container is placed which is filled with an appropriate amount of metal oxide, and where a stream of gaseous H 2 S is fed into the reactor, each reaction zone being heated to the necessary reaction temperature. h the first reaction zone conversion of elementary carbon to gaseous CS 2 takes place when gaseous H 2 S is fed over the carbon charge at an appropriate temperature. The temperature should be held at least at 900 °C; 1500 °C appears to be the upper limit for reasons of practicability. Preferably, the conversion temperature is held in the re¬ gion of 950-1000 °C. In the second reaction zone, the gaseous CS 2 which is generated in the first reaction zone reacts with the charge of metal oxide to form the desired metal sulphide. Suit¬ able reaction temperatures may be chosen between 500 °C and 1500 °C, depending on the nature and the specific requirements of the metal oxide. In most cases, reactio temperatures between 900 and 1000 °C are appropriate. Depending on the specific system, the reactivity of the oxide and the charge, one reaction run requires 6-48, preferably 12-24 hours reaction time under continuous H 2 S supply and heating for a complete conversion to the sulphide product. The reaction equipment, such as reactor tube and containers, may be made from any material having a sufficient temperature stability and which does not undergo reactions with the oxide, the carbon or the sulphiding agents under the reaction conditions applied. Reaction systems made from high temperature stable glass ware, silica or ceramics are suitable. Preferably, the reactor tube is made from silica and the containers are boats of appropriate dimensions made from silica or aluminous porcelain. The reactor tube ideally is placed in one tube furnace which allows an independent fixing and control of two temperature zones in the reactor. Alternatively, a tube reactor of suitable length is placed in two separately operated furnaces, so defining the two required reaction zones. An inlet for H 2 S and N 2 as inert gas carrier is provided at one end of the tube; the other end is connected with the exhaust- preferably provided with means for scrubbing excess H 2 S and CS 2 . A boat filled with pulverised elementary carbon is placed in the first reaction zone of the tube reactor. Elementary carbon in the form of graphite or charcoal is applicable. The charge of metal oxide is placed in another boat in the second reaction zone. For an easy conversion, the oxide should be provided in pulverised form as well. The layout of the equipment is of course dependent on the desired product output. F° r a construction which is able to deliver about 20-200 g of metal sulphide a tube reactor of about 1 m length with a diameter of about 50 to 70 mm has been found sufficient. Fig. 1 shows a sketch of a reaction equipment used in the following examples. (1) is a silica tube reactor of 1 m length and 50 mm diameter. (2) is the first reaction zone in which a boat (A) charged with pulverised elementary carbon is placed. (3) is the second reaction zone in which a boat (B) charged with pulverised metal oxide is placed. (4) is the furnace with two separately controlled temperature zones Ti and T 2 . 2 and/or H 2 S is fed into the reactor at (5). Excess gas is led to scrubber and exhaust (6). Example 1 A: AJuminous porcelain boat containing 13.5 g activated charcoal. B: Silica boat containing 41 g H0 2 O 3 . T 2 : 990 °C The reactor was brought to operating temperatures under a flow of N 2 (0.5 l/min). H 2 S was then introduced (0.2 l/min) and the N 2 shut off. Temperatures and gas flow were maintained for 48 hours, then the furnace was switched off and the reactor cooled over night in the flow of H 2 S. The reactor was purged with N 2 after turning off the H 2 S and opened up. Boat A contained 49 g holmium sulphide. Ho content = 77.5 % (theory: 77.4 %). 0 content = 0.1 % Boat B contained a small residue of charcoal. Example 2: A: Aluminous porcelain boat containing 20 g activated charcoal. Ti: 950 °C B: Silica boat containing 149 g La 2 θ 3 . T 2 : 1000 °C The reactor was brought to operating temperature under a flow of N 2 (0.4 l/min). H 2 S was then introduced (0.2 l/min). Temperatures and gas flow were maintained for 24 hours; then the furnace was switched off and the H 2 S turned off. The reactor was allowed to cool over night in the remaining flow of N 2 . The reactor was opened. Boat A contained 158 g lanthanum sulphide. La content = 74.0 % (theory: 74.3 %) S content = 25.5 % (theory: 25.7 %) O content = 0.27 % Boat B contained 2.6 g residual charcoal.

Claims: Patent Claims 1. Process for the preparation of metal sulphides by reacting corresponding metal oxides at temperatures between 500 and 1500 °C in a stream of gaseous CS 2 , characterised in that the CS 2 is generated from elementary carbon and gaseous H 2 S by reaction at temperatures between 900 and 1500 °C upstream of the metal oxide. 2. Process according to claim 1, characterised in that the reaction is carried out in a horizontally placed tube-like reactor in which in a first reaction zone a container is placed which is filled with pulverised elementary carbon and in a second reaction zone a container is placed which is filled with an appropriate amount of metal oxide, and where a stream of gaseous H 2 S is fed into the re¬ actor. 3. Process according to claim 2, characterised in that graphite or charcoal is used. 4. Use of the process according to claims 1-3 for the preparation of sulphides of Mg, Ca, Sr, Ba, Al, Ga, Si, Ge, Sn, transition metals, rare earth metals. 5. Use of the process according to claims 1-3 for the preparation of sulphides of La, Pr, Ho. 6. Use of the process according to claims 1-3 for the preparation of metal sul¬ phides with a residual oxide content not exceeding 0.5 % by weight.

Inventors: HANNEY RICHARD, KINSMAN BARRY EDWARD, HANNEY, RICHARD, KINSMAN, BARRY, EDWARD
Applicants: HANNEY RICHARD, KINSMAN BARRY EDWARD, MERCK PATENT GMBH, HANNEY, RICHARD, KINSMAN, BARRY, EDWARD
Application year: 1994
Main classification: C01B17
IPC codes: C01B17, C01F7, C01F17, C01G1, C01G15, C01G17, C01G19, C03C1